The present invention relates to a three-phase brushless motor, and more particularly it relates to a placement of stator coils of the motor.
A brushless motor (hereinafter referred to as a motor) has been required to have greater motor-constant xe2x80x9cKtxe2x80x9d representing the torque generated per unit-electric-current in order to output greater power. For getting the greater motor-constant xe2x80x9cKtxe2x80x9d, in general, winding-coils are disposed occupying an area equal to or more than an area occupied by permanent magnets (hereinafter referred to as a magnet) in a disc-shaped rotor. Then a number of coils xe2x80x9cSxe2x80x9d increases or a number of turns of each coil increases. In this case, however; if a stator is formed by disposing flat-coreless-coils on a printed circuit board, the motor becomes expensive and incurs low productivity because the cost of manufacturing equipment rises and a number of processes increase.
When motors are downsized, the area to be occupied by a bundle of winding of the coil naturally decreases. Thus the number of turns of the coil cannot be increased As a result, the motor constant cannot be greater. This problem provides users with the following alternative: One is just to persuade oneself that the downsizing of the motor entails the smaller motor constant, the other is to change windings and magnets to ones of higher performance and more expensive so that the influence, due to downsizing the motor, decreasing the motor constant can be minimized.
FIG. 2A through FIG. 2C show a conventional facing and flat three-phases brushless motor. Meanwhile, the facing and flat type is referred to as a motor structure where a rotor faces a stator via a spacing in the axial direction.
In FIG. 2B, a size of the motor is represented by diameter xe2x80x9cODxe2x80x9d extending between an outer rim of a group of the coils. When diameter xe2x80x9cODxe2x80x9d measures ca. 40 mm, another diameter xe2x80x9cIDxe2x80x9d extending between an inner rim of the group of the coils measures ca. 20 mm. In many cases, nine coils are employed by considering the balance between the radial length and circular length of the coil. In this case, since the number of polarities xe2x80x9cPxe2x80x9d of the disc-shaped rotor shown in FIG. 2C is 12, the fan-shaped polarity forms an angle of 30xc2x0 based on 360/P. The coils are arranged as shown in FIG. 2B, i.e. respective coils are spaced at intervals of 360/9=40xc2x0. Thus the space between each coil is 4/3 of the width of the polarity. In other words, the relation (coil placement condition) between the polarity width and the space between each coil is expressed as (360/P)xc3x97(4/3), with considering a placement of the polarities. Thus in this case, 40 degree interval is calculated.
In FIG. 2B, respective coils forming U, V and W phases are placed. U phase is formed by coils U1, U2 and U3. V phase is formed by coils V1, V2 and V3. W phase is formed by coils W1, W2 and W3. Nine coils in total are disposed on printed circuit board 2. Width xe2x80x9cAxe2x80x9d of winding-bundle of each coil is restricted by soldering land 4 and the adjacent coils. The soldering land 4 is disposed inside of each coil and used for terminating the coil-wire end.
Each coil, in particular, comprises numbers of winding-bundles, therefore, 0.01 mm dispersion of winding diameter causes 0.2 mm dispersion on coil""s outer diameter when the winding turns 20 ties. This dispersion and the work of fixing each coil onto the printed circuit board should be taken into consideration, the space to the adjacent coils thus should be ca. 1 mm in general. Further, another space is required for disposing magnetic sensor 5xe2x80x94a position detectorxe2x80x94for detecting a rotor position. Sensors 5 are placed inside respective three coils, i.e. coils U1, V1 and W1. The width xe2x80x9cAxe2x80x9d of winding-bundle of these three coils thus become narrower than those of other 6 coils. This restriction reduces a number of turns of these three coils, and prevents the motor constant from becoming greater. The size of the magnetic sensor and the area of the soldering land are difficult to be reduced in proportion to downsizng the motor, therefore, the influence of this restriction adversely increases more than proportional at greater downswing of the motor.
When an isosceles angle of the coil matches up to 30 degree which the rotor polarity forms, windings of the coil outwardly bulge out by ca. 1.7 mm on each side, and inwardly bulge out by ca. 0.9 mm on each side. When the space between the adjacent coil is considered, there is little space for disposing the winding outside the isosceles angle 30 degree. Therefore, if the number of turns of the coil should be increased, almost of all the windings should be disposed inside the angle 30 degree. As a result, the isosceles angle becomes practically less than 30 degree.
As such, in the case that the isosceles angle becomes smaller, one of a rotor polarity reaches the position, in a winding-bundle of one of isosceles sides, where the maximum torque is produced, then the other rotor polarity is displaced from the position, in another winding-bundle of the other side of isosceles sides, where the maximum torque would be produced. Therefore, as one entire coil, this coil cannot produce the maximum torque. As a result, the motor constant becomes smaller than the case where the isosceles angle forms 30 degree.
In the case that much more coil""s windings are disposed inside of polarity angle 30 degree, a point, where torque is produced in a direction reverse to normal rotating direction of the motor, is provided, so that the motor constant incurs some loss. This reversal point also causes vibration of the motor.
FIG. 2A illustrates this situation. FIG. 2A shows a relation between cross section of winding-bundle of the coil and a position of the rotor shown in FIG. 2B. Coil""s winding-bundles 8 and 9 are disposed with respect to polarity 7 disposed every 30 degree in rotor 6. Winding-bundles 8 and 9 form the same windings, and e.g. when electric current runs from this side to that side in bundle 8, the current runs from that side to this side in bundle 9. In other words, when different polarities face bundles 8 and 9 respectively, torque is produced in the same direction respectively; however, when the same polarities face bundles 8 and 9, torque is generated in the reverse direction and cancels each other. As shown in FIG. 2A and FIG. 2B, numbers of windings are disposed inside the 30 degree, and in this case, when rotor 6 revolves and polarity 7 reaches the position shown in FIG. 2A, Z-section of bundle 9 faces the same polarity of bundle 8. Thus reverse torque is produced in the same bundle 9.
The following prior art has been known for addressing the problem discussed above: Sensors 5 are collected and placed in the area where the coils are not disposed, as shown in FIG. 3. The coils are placed at conventional intervals of (360/P)xc3x97(4/3). For instance, 10 polarities with 6 coils can improve the problematic situation. In FIG. 3, based on the conventional coil placement condition discussed above, respective oils forming U, V and W phases are placed at intervals of 48 degree because of P=10. In other words, U phase coils are formed by coils U1 and U2, V phase coils are formed by coils V1 and V2, and W phase coils are formed by coils W1 and W2. In total 6 coils are disposed on the printed circuit board.
These 6 coils are massed in five areas disposed in every 48 degree, i.e. total area covered by 48xc3x975=240 degree, so that the space for accommodating sensors 5 can be obtained. Sensors 5 can be placed at intervals of (360/P)xc3x97(4/ 3) degree or (360/P)xc3x97(2/3) degree. The example shown in FIG. 3 shows the interval of (360/P)xc3x97(2/3) degree i.e. 24 degree.
However, when diameter xe2x80x9cODxe2x80x9d, extending the outer rim formed by each coil, is small, soldering lands 3 outside the respective coil-wire ends become closer to sensor 5. Thus when the end of each coil-wire is soldered, the coil-wire end tends to short with the terminal of sensor 5. The coil-wire end is, in particular, easy to be deformed so that it is difficult to regulate its position during the work, and the disperse of length of coil-wire end is large due to terminating work, thus the coil-wire ends need, in general, a rather longer length preliminarily. Soldering lands 3 are placed on printed circuit board 2; however, the manufacturing technique allows rather great disperse on positional accuracy of lands 3, so that ca. 0.2 mm relative displacement with sensor 5 should be allowed. Due to the reasons discussed above, enough space between land 3 and sensor 5 is needed Therefore, when diameter xe2x80x9cODxe2x80x9d is small, sensors 5 cannot be massed outside the coils, and they should be placed inside the coils instead.
In the case of the conventional motor discussed above and whose diameter xe2x80x9cODxe2x80x9d is not more than 40 mm, spaces for respective coils should be small. Further, the number of turns of the coils are difficult to increase because the space for the soldering lands and the magnetic sensors should be reserved as well as good workability should be prepared. As a result, the motor constant cannot be greater. On the contrary, the motor constant decreases more than proportional at greater downsizing of the motor when the area ratio is considered between coils"" occupying area vs. the area occupied by the magnetic sensors besides considering the dimensionally restricted soldering lands.
Other problems have been already discussed: i.e. the isosceles angle of the coil is practically smaller than the fan-shaped isosceles angle that forms the rotor polarity, so that the torque generated incurs some loss, and reverse torque is produced to counteract the torque and cause vibration of the motor.
The present invention addresses the problems discussed above, and a brushless motor of the present invention comprises the following elements:
a rotor with a permanent magnet having xe2x80x9cPxe2x80x9d (P is an integer equal to two or more than two) pieces of polarity; and
a stator facing the rotor and having a plurality of coils. The coil has isosceles sides which interlink with the magnetic field produced by the polarities, and extension lines of the isosceles sidesxe2x80x94extending through centers of winding-bundles of the coilxe2x80x94toward a shaft center cross each other at the shaft center and form an angle of 360/P degree.
This structure restrains the lowering of a motor constant due to downsizing the motor. Further, an optimum shape and optimum placement of the coils allow a number of coils to be reduced from the same size of the motor.